Function carry_unmodeled 

pub fn carry_unmodeled(
    files: &[String],
    src: &Path,
    dest: &Path,
) -> Result<Vec<String>>

Copy the named unmodeled files from src into dest, byte for byte, answering the relative paths carried. A file the source no longer holds is skipped: the list is what was seen at load, the copy is what is there now.